When a radio frequency signal is transmitted over the air from a transmitter to a receiver, it propagates along a straight line between the transmitter and receiver, as well as propagating along other paths involving reflection off objects such as buildings and mountains. As the signal travels along the different paths it becomes attenuated and delayed by different amounts, and arrives at the receiver from different angles. The propagation paths, each causing different fading of the signal, make up a radio channel along which the signal is transmitted. In addition, noise and signals from other transmitters interfere with the signal, and the total signal received at the transmitter is a sum of the faded beams of the different paths and the additional interfering signals.
Radio communication between a subscriber terminal, such as a cell phone, and a base station of a radio system can be tested using a radio channel emulator which emulates the propagation of a signal along a radio channel. The radio channel emulator simulates the fading of the signal that would be experienced by a signal being transmitted along a real channel, thereby enabling an artificially faded signal to be transmitted directly onto a device under test (DUT). By providing an artificially faded signal to a DUT, various features of the DUT may be tested, such as for example the performance of its receiver antenna configuration.
For example, with reference to FIG. 1 a channel emulator 10 may be used to generate a test signal which is amplified by a power amplifier 12 and delivered to the DUT 14 by eight transmitting probes 16 of an anechoic chamber 18. The channel emulator 10 comprises a base station emulator 20 which simulates the unfaded signal emitted by a base station, and a downlink fading emulator 22 which simulates the fading of the signal along different paths. Since each path represents reflection off a different object or ‘cluster’, two or more probes may be required to generate a beam that arrives at the DUT with the correct power angular spectrum. Thus, the probes collaborate to simulate correctly each of several incoming beams.
In the known arrangement of FIG. 1, uplink transmission from the DUT to a base station is simulated by a single uplink probe 23 and a cable 24 routed to the base station emulator 20. The uplink probe 23 receives an uplink signal radiated by the DUT and transmits this through the cable 23 directly to the base station emulator 20. The uplink signal is not passed through a fading emulator. Thus, fading of the uplink channel is not emulated in this known system, and as a result, this arrangement only provides over-the-air (OTA) testing for downlink transmission.
In another approach, it is known to enable OTA testing of a DUT by providing an anechoic chamber with OTA probes capable of both transmitting and receiving radio signals. For example, as shown in FIG. 2, an anechoic chamber 30 has eight transceiving probes 32, each of which comprises a transmitter for simulating incoming beams arriving at the DUT and a receiver for capturing signals transmitted by the DUT. In this approach, OTA fading of both the uplink and downlink channels is emulated. For the downlink channel, transmission from a base station is simulated by a base station emulator 34 of a channel emulator 36, and fading is introduced by a downlink fading emulator 38. The signals are amplified by a power amplifier 40 and routed to the probes 32 along cables 42 by a transmitter/receiver separation unit 44. For the uplink channel, signals transmitted by the DUT are captured by the probes 32 and fed along the same cables 42 to the transmitter/receiver separation unit 44. From here, the signals are amplified at a power amplifier 46 and fading is introduced by an uplink fading emulator 48.
Thus, OTA testing may be performed on a DUT for both transmitting and receiving radio signals. However, this approach only works when the uplink and downlink probes are positioned at the same locations in the anechoic chamber, and by implication when there are the same numbers of uplink and downlink probes.
However, in some circumstances there are fewer uplink probes than downlink probes. This may for example be because it is expensive to upgrade systems of the type shown in FIG. 1 by adding uplink probes and fading emulators. Furthermore, there may be fewer uplink probes by design if a subscriber terminal has fewer transmitting antennas than receiving antennas and the smaller transmitting antenna array size requires fewer uplink probes for the same accuracy of OTA testing. It is also a consideration that full duplex emulations—i.e. those simulating both uplink fading as well as downlink fading—are needed less often.
It is accordingly an object of the invention to provide an improved system for emulating an over-the-air channel for communicating with a device under test.